Extend as a PSR-0 Compliant Library

Moves `Base2n` class into a namespace and adds a `Base85` class to handle radix-85 encoding.

Also creates a factory class that can be used to quickly construct common encoding schemes.

Adds support for using `Base2n` as an API shim for native Base16 and Base64 encoding. This means it is not longer a counter example to use the class for speed.

Adds PHPT tests, which can be run with `pear` or `pyrus`. These tests check known outputs from the RFC's and other documentation for the algorithms.